Diagram of Electromigration Pattern in Eutectic Pb-Sn and Pb-Free Solders

摘要:

文章预览

Solder is the most frequently used alloy, which serves as the bonding metal for electronics components. Recently, the interconnected bump is distinctly downsizing its bulk along with the integration of high-density packaging. The evaluation of electromigration damage for solder bumps is indispensable. Hence, it is fairly urgent to understand the mechanism of the electromigration damage to be capable of securing reliability of the solder bump and ultimately predicting its failure lifetime.
Electromigration pattern in multi-phase material is determined by the combination of current density, temperature and current-applying time. In this paper, diagram of electromigration pattern (DEP) in solders is presented, where both of eutectic Pb-Sn and Pb-free solders are treated. DEP gives the basis for discussing and predicting the electromigration damage in solders.

摘要: In-situ observation by scanning electron microscope of the microstructure evolution near
the cathode depletion region and the quantitative analysis on the number of hillock phases in the
eutectic SnPb edge drift structure made it clear that the dominant migrating element and dominant
hillock phase were Sn and Pb, respectively, under 50 oC while both dominant migrating element and
dominant hillock phase were Pb above 100 oC. Such temperature-dependence of the dominant hillock
phases in the eutectic SnPb solder can be understood by considering the atomic size factors of the
metallic solid solutions.

摘要: In advanced electronic products, electromigration-induced failure is one of the most
serious problems in fine pitch flip chip solder joints because the design rule in devices requires high
current density through small solder joints for high performance and miniaturization. The failure
mode induced by electromigration in the flip chip solder joint is unique, owing to the loss of under
bump metallurgy (UBM) and the interfacial void formation at the cathode contact interface. In this
study, Electromigration of flip chip solder joints has been investigated under a constant density of
2.45×104 A/cm2 at 120 °C. The in-situ marker displacements during the electromigration test was
measured and found to show a rough linear change as a function of time. Scanning electron
microscopic images of the cross section of samples showed the existence of voids at the interface
between Al interconnection and under bump metallurgy. The void movement was matched with the
marker displacements during the electromigration test, and voids moved to the cathode interface
between Al interconnection and under bump metallurgy in the downward electron flow (from chip to
substrate) joint. The mechanism of electromigration-induced void migration and failure in the flip
chip are discussed. During electromigration, a flux of atoms is driven from the cathode to the anode or
a flux of vacancies in the opposite direction. It can lead to two possible mechanisms of void
migration. First, if we regard the void as a rigid marker of diffusion, it will be displaced towards the
cathode by the atomic flux in the electromigration, Second, if we consider surface diffusion on the
void surface, electromigration will drive atoms on the top surface of the void to the bottom surface of
the void, and consequently the void will move towards the cathode.

摘要: Both Al interconnects and flip-chip solder bumps were sensitive to high current. The failure mechanism of circuits interconnects would be more complicated if the current density in circuits was exceed the critical magnitudes of electromigration in both Al interconnects and solder bumps. The failure of circuit interconnects under different magnitudes of current density was studied and the interaction of electromigration in solder bumps and Al interconnects was discussed. The circuit interconnects of flip chip show three failure phenomena under high current density: voids in Al final metal, inter-diffusion of Al and SnPb, and melting of solder bumps. The voids in Al metal show the directional diffusion of Al atoms was mainly controlled by the electron wind fore. However the inter-diffusion of Al and SnPb demonstrated the electron wind force to Sn and Pb atoms would be ignored in contrast with chemical potential gradient or intrinsic stress. The flow of Sn and Pb atoms under high current density was in opposite direction with electron wind force and uniform with chemical potential gradient.

摘要: Electromigration effects on the solder joint formation of 99.3Sn-0.7Cu and 96.5Sn-3.0Ag-0.5Cu lead-free solder with Cu electroplated Ni layer wire were investigated. The electromigration effects on the solder joints were studied after current density stressing at 1 x 103 A/cm2 in room temperature for 0 h, 120 h, and 240 h. The research work found that intermetallic compound (IMC) formation on the joint is increases for both solders with longer period of current stress applied. Higher IMC thickness growth in 99.3Sn-0.7Cu solder joint compared to 99.3Sn-0.7Cu is detected and both anode regions of the solder joints show higher IMC thickness growth compared to cathode region. Experimental results show 99.3Sn-0.7Cu solder joint is more prone to failure under current stress compared to 96.5Sn-3.0Ag-0.5Cu solder joint with thicker IMC which translates to higher brittleness.

摘要: The technique for fabricating Al micro-materials using a conductive passivation film by electromigration (EM), which is the physical phenomenon of atomic transport with high-density electron flow, has been reported. Conductive passivation film precludes the unplanned hillock formation and substantially simplifies the sample preparation time for fabricating Al micro-materials by EM. To date, TiN that is electrical conductive material has been used as a passivation film. However, the TiN passivation oxidizes during heat and current test for fabricating Al micro-materials by EM because of inherent poor oxidation resistance of TiN. Oxidation of passivation causes a problem that applying current occasionally becomes difficult. The present paper proposes a new conductive passivation made of CrN for fabricating Al micro-materials by EM. CrN is used as a countermeasure against the oxidation problem. Additionally, the growth of Al micro-materials by EM is investigated in the relation with the experimental conditions of current and substrate temperature. As a result, we report that the fabrication of Al micro-materials using the CrN passivation is successfully demonstrated in the relation with the experimental conditions.